1. Technical Field of the Present Invention
This present invention relates to improvements in critical point dryers for sample preparation for electron microscopy and semiconductor wafer manufacturing and especially to a computer controlled critical point drying apparatus. The present invention is embodied in a method for controlling a critical point drying apparatus using a computer system, a computer system for implementing the method of controlling the critical point drying apparatus, and a computer program product bearing software instructions that implement the method of controlling the critical point drying apparatus.
2. Description of the Related Art
The following references provide useful background information on the indicated topics, all of which relate to the present invention, and are incorporated herein by reference:
U.S. Pat. No. 4,055,904 issued to Horne on Nov. 1, 1977 describes an automatic method of operating the purge and bleed modes for a critical point dryer.
U.S. Pat. No. 4,104,808 issued to Horne et al. on Aug. 8, 1978 describes a critical point dryer wherein the purge and bleed modes are controlled semi-automatically.
There will now be provided a discussion of various topics to provide a proper foundation for understanding the present invention.
In order to examine biological specimens under a scanning electron microscope, the biological specimens must be completely dried and coated with a thin conductive layer. It is important that the drying process be accomplished without disturbing the microstructure of the biological specimen to be examined. Depending upon the biological specimen""s structure, three techniques are available for drying the biological specimen. The first method is air drying by evaporation of the cellular water. While suitable for bacteria or other rigid structures, this method is detrimental to the structures of many biological specimens. The surface tension forces, which turn a grape into a raisin during the drying process, cause sufficient distortion in the cell structure of many biological specimens thereby rendering them useless. The second method is sublimation or freeze-drying. This method is useful only for very small specimens. Additionally, unless the lengthy technique is followed precisely, structural damage from thermal expansion or ice crystal formation often results. The third method utilized is the phase transitional or critical point drying which produces consistently reproducible results without the drawbacks of the preceding two methods.
Along with being used to prepare specimens for the scanning electron microscope, critical point drying may also be used in the production of MEMS (Micro- Electro-Mechanical Systems) devices. The critical point drying process helps for a sticktion free release of microstructers in the MEMS device.
In critical point drying, a dehydrating fluid such as ethanol or acetone gradually replaces the water contained in a specimen. This maintains the three-dimensional hydrated morphology of the structure under study. However, if the ethanol or acetone evaporates, surface tension forces would cause structural damage and destroy the specimen""s usefulness.
Critical point drying devices for sample preparation in electron microscopy are known in the art. The prior art critical point dryers utilize the technique of substituting a transitional fluid for the dehydrating fluid in the cell structure and then removing the transitional fluid. A critical point dryer heats and pressurizes the biological specimen until above the critical pressure and critical temperature. The critical temperature is defined as the temperature above which a gas cannot be liquefied by pressure alone. The critical pressure is the pressure that results when a substance exists as a gas and a liquid in equilibrium at the critical temperature. The critical point of a liquid is when its temperature and pressure are at or above the critical temperature and pressure and the densities of the liquid phase and vapor phase are identical. This critical point is characterized by an absence of phase boundaries that normally exist between a liquid and its vapor at temperatures and pressures below the critical point. This absence of a phase boundary eliminates the boundary forces that exist when changing a liquid to a gas. These boundary forces often cause the destruction of the extremely delicate specimens when changing its internal liquid to a gas below the critical point. Therefore, the solution which is applied in a critical point drying process is to remove the internal liquid from the biological specimen above its critical pressure and temperature to eliminate the boundary force destruction that would otherwise result.
Although all fluids have a characteristic critical point which should allow direct removal without the use of dehydrating or transitional fluids, the critical point temperature and pressure of water is 374.2xc2x0 C. and 218 atmospheres. Achieving these temperatures and pressures would cause severe damage to most biological specimens and therefore a fluid having a lower critical temperature and pressure is normally substituted. Commonly, a dehydrating fluid is used that is miscible with water (e.g., ethanol or acetone) as an intermediate stage between the specimen containing water and a specimen containing transitional fluid.
Typically, and in the prior art dryers, the transitional fluid commonly used is carbon dioxide (CO2) because it is easy to use, more economical, less noxious and provides consistently better results than other transitional fluids. The critical temperature and pressure of carbon dioxide is 31xc2x0 C. and 1,072 psi, respectively, thus reducing the potential for destruction of the specimen structure.
The known instruments and apparatuses for critical point drying of biological specimens include, of course, a drying chamber that is connected a supply of the transitional fluid with various regulating valves, temperature gauges and a means for heating the chamber. A skilled technician must carefully control the application, heating, pressurizing and removal of the transitional fluid, thus requiring not only time but also constant attention. Applicants are unaware of a computer-controlled critical point drying apparatus that eliminates the need for constant operator attention.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and limitations of the prior art.
Additional objects and advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
According to a first aspect of the invention, a critical point drying chamber for drying specimens comprising a chamber and at least one heater is provided. The apparatus further comprises a first valve assembly that supplies a cooling fluid to the chamber. The apparatus further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the chamber. The apparatus further comprises a third valve assembly that allows an intermediary fluid to be purged from the chamber. The apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the chamber. The apparatus further comprises a condenser collector that collects intermediary fluid exiting the third valve assembly.
According to a second aspect of the invention, a critical point drying apparatus for drying specimens comprises a cylindrical drying chamber having concave end portions and at least one heater. The apparatus further comprises a first valve assembly that supplies a cooling fluid to the drying chamber. The apparatus further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the drying chamber. The apparatus further comprises a third valve assembly that allows an intermediary fluid to be purged from the drying chamber. The apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the drying chamber.
According to a third aspect of the invention, a critical point drying apparatus for drying specimens comprises a drying chamber having at least one heater. The apparatus further comprises a first valve assembly that supplies a cooling fluid to the drying chamber. The apparatus further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the drying chamber. The apparatus further comprises a third valve assembly that allows an intermediary fluid to be purged from the drying chamber. The apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the drying chamber. The apparatus further comprises a computer system that operates the first, second, third and fourth valve assemblies and that activates the at least one heater to heat the transitional fluid above the critical point temperature and to pressurize the transitional fluid above the critical point pressure.
According to a fourth aspect of the invention, a critical point drying apparatus for drying specimens, comprising a drying chamber having concave end portions and at least one heater is provided. The apparatus further comprises a first valve assembly that supplies a cooling fluid to the drying chamber wall. The apparatus further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the interior of the drying chamber. The apparatus further comprises a third valve assembly that allows an intermediary fluid to be purged from the interior of the drying chamber. The apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the interior of the drying chamber at a predetermined rate. The apparatus further comprises a computer system that operates the first, second, third and fourth valve assemblies and that activates the at least one heater to heat the transitional fluid above the critical point temperature and to pressurize the transitional fluid above the critical point pressure.
According to a fifth aspect of the invention, a critical point dryer apparatus comprising a drying chamber and a computer system wherein the computer system is adapted to controlling the drying chamber during a critical point drying process is provided. The computer system comprises a processor and a memory including software instructions adapted to enable the computer system. The software instructions enable the computer system to cool the drying chamber to a first chamber temperature. The software instructions further enable the computer system to fill the drying chamber with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber at the first chamber temperature such that the transitional fluid completely displaces an intermediary fluid within a first time period. The software instructions further enable the computer system to activate at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching critical point equilibrium. The software instructions further enable the computer system to maintain the transitional fluid at the critical point equilibrium for a second time period. The software instructions further enable the computer system to bleeding the transitional fluid from the drying chamber while maintaining the drying chamber at the second chamber temperature and allowing the drying chamber pressure to drop.
According to a sixth aspect of the invention, a computer program product for enabling a computer system to control the drying chamber during a critical point drying process is provided. The computer program product comprises software instructions for enabling the computer system to perform predetermined operations, and a computer readable medium bearing the software instructions. The predetermined operations comprise cooling the drying chamber to a first chamber temperature. The predetermined operations comprise filling the drying chamber with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber at the first chamber temperature such that the transitional fluid completely displaces the intermediary fluid within a first time period. The predetermined operations comprise activating at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching critical point equilibrium. The predetermined operations comprise maintaining the transitional fluid at the critical point equilibrium for a second time period. The predetermined operations comprise bleeding the transitional fluid from the drying chamber while maintaining the drying chamber at the second chamber temperature.
According to a seventh aspect of the invention, an article of manufacture, which comprises a computer readable medium having stored therein a computer program to control a drying chamber during a critical point drying process, is provided. The article of manufacture comprises a first code segment which, when executed on a computer, cools the drying chamber to a first chamber temperature. The article of manufacture comprises a second code segment which, when executed on a computer, fills the drying chamber with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber at the first chamber temperature such that the transitional fluid completely displaces the intermediary fluid within a first time period. The article of manufacture comprises a third code segment which, when executed on a computer, activates at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching a critical point equilibrium. The article of manufacture comprises a fourth code segment which, when executed on a computer, maintains the transitional fluid at the critical point equilibrium for a second time period. The article of manufacture comprises a fifth code segment which, when executed on a computer, bleeds the transitional fluid from the drying chamber while maintaining the drying chamber at the second chamber temperature.